efectos de radiación solar en fertilidad y el horario de apertura de flor en arroz en condiciones...

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Effects of Solar Radiation onFertility and the Flower Opening Timein Rice Under Heat Stress Conditions

Kazuhiro KobayasiShimane University

Japan

1. Introduction

This chapter focuses upon the effects of solar radiation on the flower opening time andpanicle temperature, both of which significantly affect heat-stress-induced sterility in rice,although solar radiation affects crop growth rates and production through photosynthesis,the fundamental physiological function of green plants. Three possible traits that maymitigate the damage caused by high temperature during the flowering period in rice havebeen proposed (Zhao et al., 2010). The first trait is the formation of a long dehiscence at thebase of the anther, but this trait is under genetic control so that it is not affected by solarradiation. The second trait is early morning flower opening to avoid high temperaturesduring anthesis. The flower opening time can be affected by solar radiation (Kobayasi et al.,

2010). The third trait is panicle temperature, which can also be affected by solar radiation aswell as microclimates and cultivar-related factors such as transpirational conductance andpanicle shape (Yoshimoto et al., 2005). The latter two traits are discussed in this chapter.

Two experiments were conducted in Shimane Prefecture, Japan to reveal the roles of solarradiation in determining the flower opening time and panicle temperature. In section 2, theeffects of solar radiation on the flower opening time have been examined. Early morning openingof rice flowers is a beneficial response to avoid heat-stress-induced sterility because the sensitivityof rice flowers to high temperatures decreases during the 1-hr period after flower opening(Satake & Yoshida, 1978). The effects of solar radiation on the flower opening time wereevaluated using generalized linear models. Cultivar differences in the contribution of solar

radiation to the flower opening time were estimated. Furthermore, the roles of the diurnal changein solar radiation on determining the flower opening time were examined. In section 3, the effectsof solar radiation on panicle temperature have been examined. Atmospheric temperature istypically used to estimate temperature-dependent stress. However, solar radiation also affectsplant tissue temperature. An empirical model to estimate the contribution of solar radiation topanicle temperature was developed using generalized linear models.

2. Experiment 1: Effects of solar radiation on the flower opening time in rice

Global climate change poses a serious challenge to crop production around the world. In rice(Oryza sativa L.), temperatures higher than 34 C at the time of flowering may induce
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flower sterility and decrease yields, even in temperate regions such as southern Japan, if thecropping season is not changed to avoid such temperatures (Horie et al., 1996; Kim et al.,1996). Crop simulation models (Horie et al., 1996) have suggested that yields of currentlygrown rice varieties in southern Japan would be reduced by up to 40% under future climate

scenarios. Serious yield losses in rice due to flower sterility occurred in the Yangtze Valley ofChina in 2003, when the temperatures during the hottest summer in the regions historyaffected the reproductive stage of rice cultivated in this region (Wang et al., 2004).

Early morning opening of rice flowers is a beneficial response to avoid sterility caused byheat stress during anthesis because the sensitivity of rice flowers to high temperaturesdecreases during the 1-hr period after flower opening (Satake & Yoshida, 1978). Thus, aflower opening time, 1 hr earlier than normal may reduce the risk of sterility because it maylead to anthesis before the air temperature reaches the critical level; air temperature can riseat a rate of higher than 3 C hr1 starting at approximately 1000 (Nishiyama & Blanco, 1980).A controlled environment experiment revealed that flowers of Milyang 23 that opened

earlier in the morning and at a lower temperature had higher seed sets than those thatopened later (near midday) and at higher temperatures (Imaki et al., 1987).

Thus, the selection of cultivars with early flower opening times is an important method forreducing heat-induced sterility (Ishimaru et al., 2010; Jagadish et al., 2008; Nishiyama &Blanco, 1980). For example, the flowers of O. glaberrima Steud. open earlier than those of O.sativa L. (Jagadish et al., 2008; Nishiyama & Blanco, 1980), and the flowers of interspecifichybrids between O. glaberrima and O. sativa open earlier than those of O. sativa (Nishiyama &Satake, 1981). Reduced sterility in the early morning flowering line subjected to risingtemperatures during anthesis in the greenhouse was attributed to an earlier flowering timecompared with Koshihikari (Ishimaru et al., 2010).

Although the flower opening time is under genetic control, it is affected by aspects of theweather such as solar radiation and air temperature (Hoshikawa, 1989; Imaki et al., 1983;Jagadish et al., 2007, 2008; Kobayasi et al., 2010; Nakagawa & Nagata, 2007; Nishiyama &Satake, 1981). The relationship between the flower opening time and solar radiation has beenresearched under field conditions (Kobayasi et al., 2010). Using correlation analysis, mostjaponica cultivars showed a negative correlation between solar radiation and flower openingtimes; this correlation showed that higher solar radiation resulted in earlier flower openingtimes. However, the indica cultivar IR72 did not show a negative correlation between thesolar radiation and flower opening time. This result suggests that the contribution of solarradiation to the flower opening time is different among ecotypes, and compared withjaponica cultivars, indica cultivars have lower sensitivity in determining the flower opening

time by solar radiation. Moreover, the response of flower opening to high temperature differsamong rice cultivars. The flower opening time occurs earlier at high temperatures inMilyang 23, whereas it occurs later in 'Nipponbare' (Imaki et al., 1983). In a study of indicacultivars, the flower opening time occurred approximately 45 min earlier at highertemperatures (Jagadish et al., 2007). Furthermore, the relationship between solar radiationand air temperature should be incorporated into the analysis of the flower opening timebecause the amount of solar radiation is one of the most important factors in determining airtemperature. It has been suggested that synergistic effects on the flower opening time mayexist between temperature and light (Kobayasi et al., 2010). In addition to solar radiation andair temperature, other weather factors such as vapor-pressure deficit and wind speed(Tsuboi, 1961) affect the flower opening time. However, the combined effects of


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Effects of Solar Radiation on Fertilityand the Flower Opening Time in Rice Under Heat Stress Conditions 247

air temperature, solar radiation, vapor-pressure deficit, and wind speed on the floweropening time of various rice genotypes remain unclear, particularly under field conditions;this limits our ability to predict the flower opening time.

The cycle of solar radiation may also affect the flower opening time as well as the amount ofsolar radiation. Most studies on the effects of weather factors on the flower opening timehave been conducted under controlled environments with artificial light conditions (Imaki etal., 1983; Jagadish et al., 2007, 2008; Nishiyama & Blanco, 1981) and the flowers of rice plantsgrown in a glasshouse or a growth chamber have been reported to open 12 hr later thanthose grown outdoors (Imaki et al., 1982). This suggests that solar radiation substantiallyaffects the flower opening time; not only the strength of solar radiation but also lightconditions can influence the flower opening time because the duration of anthesis increasesunder continuous light or dark conditions (Hoshikawa, 1989). The light intensity and cycle oflight and dark may affect the flower opening time. The effects of a diurnal cycle of light onthe flower opening time in dicotyledonous Pharbitis nil flowers have been experimentally

studied under artificially controlled conditions (Kaihara & Takimoto, 1979).In this section, first, the role of solar radiation in determining the flower opening time wasevaluated using correlation analysis between solar radiation and the flower opening time.The correlations between the flower opening time and solar radiation averaged hourly, from0500 to 1000 were analyzed. Second, we used general linear models to separately evaluate theeffects of the type of cultivars, air temperature, solar radiation, vapor-pressure deficit, andwind speed on the flower opening time. In the second analysis, we used two types of 1-hrtime spans: a 1-hr time span based on Japan Standard Time, and a 1-hr time span based onthe time of mean flower opening times in each cultivar; this eliminated the effects of thecircadian rhythm. Finally, we attempted to detect the roles of the diurnal change in solarradiation in determining the flower opening time.

2.1 Materials and methods

Experiment 1 was conducted on an experimental field of Shimane University in Matsue,Shimane Prefecture, Japan (3529N, 13304E, 4 m above sea level) in 2010. Three indica cultivarsand four japonica cultivars with wide ranges of flower opening times (Kobayasi et al., 2010) wereused (Table 1). Xiaomazhan had the earliest flower opening time in Japan (Kobayasi et al., 2010)and in Jiangsu Province, China (Zhao et al., 2010) among indica and japonica cultivars. Amongjaponica cultivars, Milky Queen had the earliest flower opening time in Japan (Kobayasi et al.,2010) and in Jiangsu Province, China (Zhao et al., 2010). Although indica cultivars are notcommonly planted in Japan, we used IR72 and Takanari because the flowers of indica cultivars

open earlier than those of japonica cultivars (Imaki et al., 1987).

The soil type at the study site was alluvial sandy clay. On three occasions, 30-day-oldseedlings grown in nursery boxes were manually transplanted to the experimental field ofShimane University to obtain flowering plants under different weather conditions. Table 1shows the seeding dates, ecotype, origin, and measurement periods for the flower openingtime and weather factors. The planting density was 22.2 hills m2 (one seedling per hill, 15-cmhill spacing, and 30-cm row spacing). The area of each plot was 12.8 m2. A basal dressing of4.0 g m2 of N, 4.9 g m2 of P2O5, and 4.3 g m

2 of K2O was applied. Top dressing was not used.Culture methods such as irrigation and pesticide used followed the standard local practicesfor rice production in Shimane Prefecture.


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248 Solar Radiation

Cultivar Ecotype Origin Seeding dates Measurement period

Fujihikari japonica JapanApril 15,

May 7July 1320, 2327

Koshihikari japonica JapanApril 15,May 7, 28

July 2631,August 511, 1721

Xiaomazhan indica ChinaApril 15,May 7, 28

July 2529,August 37, 2025

Milky Queen japonica Japan May 7, 28 August 511, 1721

IR72 indica Philippines May 7, 28August 2429,September 511

August 28

a anari in ica apan ay September 2

Asahi japonica Japan May 7, 28 September 29

Table 1. Ecotype, origin, seeding dates and measurement periods for the seven O. sativacultivars used in Experiment 1.

2.1.1 Measurements of the flower opening time and weather factors

Physical stimuli such as touch may promote flower opening in rice (Tsuboi, 1961). To avoidthis phenomenon, we used digital photographs of the panicles instead of physical

inspections. The panicles were photographed at 10-min intervals with waterproof digitalcameras (Optio W30, Pentax, Tokyo, Japan) to determine the flower opening time of theseven cultivars. The photographs were automatically taken using cameras. We put thecamera on a tripod and used a built-in electronic timer to control the measurement intervals.We recorded the time of anther extrusion of all observable flowers (more than 30% of flowersin a panicle) on the obverse side of approximately 10 panicles per day per cultivar. Themedians of anther extrusion time among all observed flowers in each panicle werecalculated. The flower opening time was defined as the mean of the medians per day.Recording the anther extrusion time of the flowers behind panicles and leaves wasoccasionally difficult.

We measured air temperature, solar radiation, relative humidity, and wind speed every 5min using a wireless weather station (Wireless Vantage Pro, Davis Instruments, Hayward,CA, USA), which was located at the experimental field of Shimane University(http://www.ipc.shimane-u.ac.jp/weather/station/i/home.html). The weather station wasinstalled at a distance of approximately 5 m from observation plots. The ground surfacebelow the station was covered with grass. We installed a thermometer, hygrometer,solarimeter, barometer, and an anemometer at heights of 150, 150, 180, 150, and 300 cm,respectively. At our study site, the sun rose between 0502 (July 13) and 0547 (September 11);thus, we did not measure solar radiation before twilight. Vapor-pressure deficits werecalculated from air temperature and relative humidity using the method based onmicroclimate (Buck, 1981).
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Effects of Solar Radiation on Fertility

and the Flower Opening Time in Rice Under Heat Stress Conditions 249

2.1.2 Statistical analysis

Pearsons correlation analysis was used to identify the relationship between solar radiation andthe flower opening time. In this analysis, hourly average of solar radiation values between 0500

and 1000 were used. Correlation analysis was restricted to the period between 0500 and 1000because sunrise hours during the flower opening time observation periods were around 0500,and flowers of Xiaomazhan usually started to open before 1000. It is possible that the span inwhich flowers respond to solar radiation before flower opening are different among cultivars,i.e. cultivars with early morning flowering would respond earlier in the morning to solarradiation. Hourly average of solar radiation values were used over seven 1-hr periods based onthe mean flower opening time in each cultivar (successive 1-hr periods from 7 hr before themean flower opening time until 1 hr before the mean flower opening time).

The collected data were analyzed by means of generalized linear models and multipleregression procedures using SPSS (Version 14J for Windows, SPSS Japan Inc., Tokyo, Japan).Because inherent relationships exist among weather factors, relatively high correlations may

exist among solar radiation, air temperature, vapor-pressure deficit, and wind speed.Therefore, to evaluate their individual effects as well as cultivar effects on the flower openingtime, generalized linear models were used. In this analysis, solar radiation, air temperature,vapor-pressure deficit, and wind speed values were averaged over five 1-hr periods (05000600, 06000700, 07000800, 08000900, and 09001000). The relative contribution of eachweather component to the flower opening time was determined using their standardizedpartial regression coefficients, and the overall strength of the relationships was quantifiedusing the multiple correlation coefficients. The contribution of solar radiation, airtemperature, vapor pressure deficit, and wind speed to the flower opening time in eachcultivar was estimated by substituting the obtained weather data in the generalized linearmodels, and the relative contributions of solar radiation, air temperature, vapor pressure

deficit, and wind speed to the flower opening time among the cultivars were evaluated. Toexamine the role of diurnal changes in solar radiation in determining the flower openingtime, multiple correlation coefficients from 1800 on the day before flowering to 0900 on theflowering day were calculated for each cultivar.

2.2 Results

In 2010, record heat and sunshine occurred in Japan because a strong North Pacific High coveredthe country during summer. The total solar radiation between 0000 and 1200 during the floweropening time observation periods ranged from 1.0 to 13.3 MJ m2. Sunshine hours in the formerand latter halves of August 2010 were 6.58 and 9.75, respectively. The normal sunshine hour in

August is 6.47. This suggests that average solar radiation during the flower opening timeobservation periods was higher than usual in that year. The vapor-pressure deficit between 0000and 1200 during the flower opening time observation period ranged from 1.9 to 18.3 hPa.

The maximum and minimum air temperature between 0000 and 1200 during the floweropening time observation period ranged from 24.6 to 36.7C and from 19.4 to 27.3C,respectively (Fig. 2). In particular, the mean air temperatures between the latter part of Julyand August in 2010 reached record-high levels. The maximum and minimum airtemperature increased rapidly from mid-July through early August. The maximum airtemperature was higher than 34C (temperatures >34C at the time of flowering may induceflower sterility and decrease yield) for 23 days, however, in normal years, the maximum airtemperature was rarely higher than 34C.


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250 Solar RadiationSolar radiation

Vapor pressure difference

Solarradiation(MJ

m-2)

1 4

1 2

1 0

8

6

4

2

0

1 6

1 4

1 2

1 0

8

6

4

2

0Vaporpressurediffere

nce(hPa)

J u l y A u g u s t S e p t e m b e r

Temperature(oC)

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Flower opening time observation period

Fig. 1. Total solar radiation and average vapor-pressure deficit between 0000 and 1200

during the observation period.

J u l y A u g u s t S e p t e m b e r

Flower opening time observation period

Maximum temperature

Minimum temperature

Fig. 2. Maximum and minimum temperatures between 0000 and 1200 during theobservation period.

Average wind speed and atmospheric pressure ranged from 0.0 to 1.4 m s

1 and from 1001 to1017 hPa, respectively, between 0000 and 1200 during the flower opening time observationperiod. The wind speed and atmospheric pressure were stable because a strong North PacificHigh covered Japan during the summer of 2010.

Significant correlations existed between the meteorological variables. In particular, thecorrelation between air temperature and vapor-pressure deficit was high (r = 0.715, p

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